#include<stdio.h>
#include<math.h>
int  main()
{
	double a,b,c,t;
	scanf("%lf %lf %lf",&a,&b,&c);
    	t=(b*b+c*c-a*a)/(2*b*c);
     double  temp=acos(t);
	printf("%0.3lf\n",temp);
	return  0;
}
